When I Link Some Modifiers From One Object To Another It Dosen't Copy All of Them
This was reported to the bug tracker, here: T66798

It was concluded that it was intentional that it works this way and was therefore not a bug. 

Philipp Oeser had this to say:


  
  from the Physics panel, Forcefield, Ridgid Body, Ridgid Body Constraint are no real modifiers, so these wont be copied
  Collision is explicitly excluded in code [this could be made working, needs some care because data related to collisions is stored
  in PartDeflect which has other stuff in it too, cant just copy it,
  will look into it but that's for later...]
  the others seem to be working fine
  so no bug here
  


Essentially, this seems to be saying that the Collision modifier is skipped because there is other data that can't be copied in a simple way. So, rather than a bug, it seems to be a limitation which may or may not be improved in the future.

As to why any existing modifiers are removed from the object you want to copy modifiers to, existing modifiers are replaced, not added to. So any existing modifiers are removed before any modifiers that can be copied are added.